Rhythmic contractions of a healthy heart are normally controlled by the sinoatrial (SA) node that includes specialized cells located in the superior right atrium. The SA node is the normal pacemaker of the heart, typically initiating 60–100 heartbeats per minute. When the SA node is pacing the heart normally, the heart is said to be in normal sinus rhythm (NSR).
The heart has specialized conduction pathways in both the atria and the ventricles that enable the rapid conduction of excitation impulses (i.e. depolarizations) from the SA node throughout the myocardium. These specialized conduction pathways conduct the depolarizations from the SA node to the atrial myocardium, to the atrio-ventricular node, and to the ventricular myocardium to produce a coordinated contraction of both atria and both ventricles.
The conduction pathways synchronize the contractions of the muscle fibers of each chamber as well as the contraction of each atrium or ventricle with the contralateral atrium or ventricle. Without the synchronization afforded by the normally functioning specialized conduction pathways, the heart's pumping efficiency is greatly diminished. Patients who exhibit pathology of these conduction pathways can suffer compromised cardiac output, such as that associated with congestive heart failure, for example.
Cardiac rhythm management devices have been developed that provide pacing stimulation to one or more heart chambers in an attempt to improve the rhythm and coordination of atrial and/or ventricular contractions. Cardiac rhythm management devices may incorporate defibrillation and/or pacemaker circuitry used to treat patients with serious arrhythmias. Cardiac rhythm management devices typically include circuitry to sense signals from the heart and a pulse generator for providing electrical stimulation to the heart. One or more leads are typically delivered transvenously or transthoracicly into the heart, and are coupled to electrodes that contact the myocardium for sensing the heart's electrical signals and for delivering stimulation to the heart in accordance with various therapies. Cardiac rhythm management devices may deliver low energy electrical pace pulses timed to assist the heart in producing a contractile rhythm that maintains cardiac pumping efficiency appropriate to meet the metabolic requirements of the patient.
While transvenous lead delivery is appropriate for many patients that experience adverse synchronization conditions, there are a significant number of patients who could benefit from cardiac resynchronization therapy or other cardiac stimulation therapies, but are not good candidates for transvenous surgical procedures. Many of these patients are considered poor candidates for transvenous lead implantation for various reasons, including inability to locate the coronary sinus, presence of coronary sinus stenosis, inability to catheterize a desired branch vein, instability of the transvenous lead, or unacceptably high pacing threshold, for example.